1. Field of the Disclosure
This disclosure pertains to microelectronic circuits, and more specifically to low voltage and high power-supply-rejection ratio (PSRR) voltage-controlled oscillators (VCOs).
2. Description of the Related Art
Voltage controlled oscillators (VCOs) are electronic circuits having the output oscillation frequency that is a function of the input voltage. VCOs are widely used in many applications such as various communication circuits to generate synchronization signals. For example, VCOs are important basic building blocks in phase lock loops (PLL) that generate an output signal of which the phase is related to the phase of the input signal. VCOs are needed to generate clock frequencies for different functional blocks such as general purpose processor/memory clocks, analog to digital and digital to analog conversion clocks, digital signal processing clocks, etc.
To maintain proper signal integrity in these applications, VCOs' outputs need to exhibit sufficiently low jitter. However, power supplies are not noise free and VCOs tend to be sensitive to noise in the supply voltage. VCOs with low power-supply-rejection ratio (PSRR) may cause excess jitter in the VCO output and fail to meet clock timing-jitter requirements. For example, such timing-jitter may increase the bit error-rate of a communication system in which the VCO is implemented. In addition, the VCO output may be subject to excessive phase noise and violate clock phase-noise requirements. Phase noise refers to random fluctuations in a clock signal and may cause cross-talk or increased bit error-rates in communications systems where the VCO is implemented.